1. Field of the Invention
The present invention relates to methods and apparatus for controlling the image output of a video camera. More particularly, this invention pertains to a method and apparatus for regulating the image generated by a video camera of the charge coupled device (CCD) type.
2. Description of the Prior Art
The field of video endoscopy to which the present invention generally relates includes medical diagnostic and therapeutic disciplines that utilize endoscopes to penetrate and to view otherwise-inaccessible body cavities with minimal intrusion and surgical procedures. Conventional endoscopes can generally be categorized into two classes, namely rigid and flexible. Examples include the laparoscope, cystoscope, arthroscope, ureterscope, bronchoscope and colonoscope.
While providing a substantial technical advance, the utility of endoscopic instruments has been significantly increased by the development of video display processing. Video information display protects the vision of the physician, particularly in those instances where a highly-reflective medium is viewed under bright illumination. This often occurs in arthroscopic surgery where the high-intensity illumination of reflective articular tissue can cause injury to the retina.
The coupling of a video camera to an endoscopic probe, as opposed to direct viewing, promotes operator comfort and, hence, instrument utility. When using a camera, the physician needn't continually reposition himself at an eyepiece near the proximal end of the probe. An assistant can hold and position the instrument, leaving the operating physician's hands free to manipulate surgical tools. As the doctor and assistant can both view the image on a common monitor, prompt and accurate movement of the scope is assured throughout. Thus, tissue trauma is minimized.
The incorporation of a video camera into an endoscopic system permits both recordation and real time transmission of procedures, opening up many possibilities not available to conventional endoscopy including real time consultations (and teaching) at distant venues and significant documentation benefits.
The numerous benefits of video information display of data gathered through endoscopic examination depend, of course, upon the diagnostic quality of the resultant image. The level of illumination can vary dramatically as the distal end of the scope is manipulated during an endoscopic examination due to the geometrical relationship between light intensity and distance.
The standard method for controlling the exposure of a conventional video camera that must process video images subject to large "swings" in intensity is the diaphragm-like iris. Unfortunately, an iris is not appropriate for an endoscopic camera. Due to obvious physical constraints, the image diameter delivered through an endoscope must be relatively small (approximately one to two millimeters in diameter). Accordingly, the iris of an endoscopic camera would have to be controllable over a very small operating range. The design and maintenance of such a precision iris, if possible, would be an extremely difficult mechanical design task. In addition, the iris would require a waterproof environmental enclosure to allow for immersion in sterilizing fluids, further complicating design.
Due to the inherent difficulty of adapting a conventional iris to the above-described problems of the environment, endoscopic cameras conventionally rely upon an internal automatic gain control (AGC) to modulate the amplification of the signal amplitude that is output from its CCD's image collector sensors. Such an AGC is generally usefully capable of operation over a range of 4:1 or perhaps 8:1. In many cases this is inadequate to compensate for the vast differences in illumination that are encountered during an examination or procedure. Furthermore, an AGC can in no way compensate for a condition of saturation in the CCD where the capacity of photo sites to linearly convert light to electrons has been exceeded. As a result, the video image can obscure features of diagnostic or other significance.